Move Maps' back pointers from "transitions" to "constructor" field

src/heap-snapshot-generator.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/heap-snapshot-generator-inl.h"
 
 #include "src/allocation-tracker.h"
 #include "src/code-stubs.h"
@@ skipping 1271 matching lines @@
     STATIC_ASSERT(Context::FIRST_WEAK_SLOT + 4 ==
                   Context::NATIVE_CONTEXT_SLOTS);
   }
 }
 
 
 void V8HeapExplorer::ExtractMapReferences(int entry, Map* map) {
   if (map->HasTransitionArray()) {
     TransitionArray* transitions = map->transitions();
     int transitions_entry = GetEntry(transitions)->index();
-    Object* back_pointer = transitions->back_pointer_storage();
-    TagObject(back_pointer, "(back pointer)");
-    SetInternalReference(transitions, transitions_entry,
-                         "back_pointer", back_pointer);
 
     if (FLAG_collect_maps && map->CanTransition()) {
       if (!transitions->IsSimpleTransition()) {
         if (transitions->HasPrototypeTransitions()) {
           FixedArray* prototype_transitions =
               transitions->GetPrototypeTransitions();
           MarkAsWeakContainer(prototype_transitions);
           TagObject(prototype_transitions, "(prototype transitions");
           SetInternalReference(transitions, transitions_entry,
                                "prototype_transitions", prototype_transitions);
         }
         // TODO(alph): transitions keys are strong links.
         MarkAsWeakContainer(transitions);
       }
     }
 
     TagObject(transitions, "(transition array)");
-    SetInternalReference(map, entry,
-                         "transitions", transitions,
-                         Map::kTransitionsOrBackPointerOffset);
-  } else {
-    Object* back_pointer = map->GetBackPointer();
-    TagObject(back_pointer, "(back pointer)");
-    SetInternalReference(map, entry,
-                         "back_pointer", back_pointer,
-                         Map::kTransitionsOrBackPointerOffset);
+    SetInternalReference(map, entry, "transitions", transitions,
+                         Map::kTransitionsOffset);
   }
   DescriptorArray* descriptors = map->instance_descriptors();
   TagObject(descriptors, "(map descriptors)");
   SetInternalReference(map, entry,
                        "descriptors", descriptors,
                        Map::kDescriptorsOffset);
 
   MarkAsWeakContainer(map->code_cache());
   SetInternalReference(map, entry,
                        "code_cache", map->code_cache(),
                        Map::kCodeCacheOffset);
   SetInternalReference(map, entry,
                        "prototype", map->prototype(), Map::kPrototypeOffset);
-  SetInternalReference(map, entry,
-                       "constructor", map->constructor(),
-                       Map::kConstructorOffset);
+  Object* constructor_or_backpointer = map->constructor_or_backpointer();
+  if (constructor_or_backpointer->IsMap()) {
+    TagObject(constructor_or_backpointer, "(back pointer)");
+    SetInternalReference(map, entry, "back_pointer", constructor_or_backpointer,
+                         Map::kConstructorOrBackPointerOffset);
+  } else {
+    SetInternalReference(map, entry, "constructor", constructor_or_backpointer,
+                         Map::kConstructorOrBackPointerOffset);
+  }
   TagObject(map->dependent_code(), "(dependent code)");
   MarkAsWeakContainer(map->dependent_code());
   SetInternalReference(map, entry,
                        "dependent_code", map->dependent_code(),
                        Map::kDependentCodeOffset);
 }
 
 
 void V8HeapExplorer::ExtractSharedFunctionInfoReferences(
     int entry, SharedFunctionInfo* shared) {
@@ skipping 1783 matching lines @@
   writer_->AddString("\"<dummy>\"");
   for (int i = 1; i < sorted_strings.length(); ++i) {
     writer_->AddCharacter(',');
     SerializeString(sorted_strings[i]);
     if (writer_->aborted()) return;
   }
 }
 
 
 } }  // namespace v8::internal